The present invention relates to a receiver and a frequency deviation measuring unit which are suitable for measuring the position of a terminal node having radio communication capabilities, and a positioning and ranging system which employs the receiver and frequency deviation measuring unit.
Public attention has been attracted by a wireless sensor network (hereinafter simply called the “sensor net”) which includes devices having sensing capabilities installed everywhere around a region to form a network over the air, thereby efficiently capturing information in the real world into an information network such as the Internet. In concept, the sensor net is autonomously built by an infinite number of nodes (terminals), each of which comprises a sensor, a microcomputer, a radio communication device, and a power supply and measures situations relating to persons, objects, environment and the like by the sensor. The sensor net is now under investigation for applications to a variety of fields such as distribution, automobile, agriculture and the like.
To implement a sensor net, nodes must be installed in a region under monitoring to continuously sense the state in the region for a long time. For this purpose, the node is required to be small in size and to consume least possible power. Also, since a large number of nodes are distributed, the management of the nodes is a critical aspect.
Likewise, low power radio communication technologies are required for the sensor net. An ultra-wide band (hereinafter abbreviated as “UWB”) communication device is expected for use in the sensor net because of its possible low power consumption and small size. The UWB radio communication is defined to employ radiowaves in a bandwidth equal to or larger than 500 MHz with the ratio of the bandwidth to the center frequency of 20% or more. The UWB communication spreads data over an extremely wide frequency band for transmission and reception, and requires extremely small signal energy per unit frequency band. Accordingly, the UWB communication can communicate without interfering with other communication systems, and can share a frequency band.
Moe Z. Win et al, “Impulse Radio: How It Works,” IEEE Communications Letters, Vol. 2, No. 2, pp. 36-38 (February 1998) discloses an example of the UWB communication which is an UWB-IR (Ultra Wide Band—Impulse Radio) communication system that modulates Gaussian mono-pulses in accordance with a PPM (Pulse Position Modulation) scheme. For establishing the synchronization with such a pulse signal, for example, a method is known to shift a timing at which a template pulse is generated at predetermined intervals to find a correlation (see, for example, JP-A-2004-241927).
Because of its abilities to use pulse signals, the UWB is known to be capable of highly accurate position measurements. For example, JP-A-2004-258009 discloses a ranging/positioning system which utilizes a transmission of packets and associated response procedure between two radios for ranging and positioning. JP-A-2004-254076 discloses a positioning system which comprises a receiver for detecting a change in the distance between a transmitter and a receiver based on a timing adjusting amount when a received pulse waveform is correlated to a template waveform to establish the synchronization.
Also, in a known node position measuring system, a signal from a node is received at a plurality of access points to calculate the position of the node, utilizing time differences of arrival (TDOA) among the access points. For example, JP-A-2005-140617 discloses a positioning method, where a plurality of access points measure a reception time difference between a positioning signal from a node and a reference signal from a reference station to position the node based on the reception time differences utilizing the TDOA.
One of challenges in positioning/ranging systems is improvements in positioning accuracy. The system described in JP-A-2004-258009 requires a high-speed oscillator and a high-speed counter in order to improve the ranging accuracy. Also, while a transmitter and a receiver comprise different clock generators for generating clocks at predetermined frequencies, respectively, the ranging accuracy is affected by the accuracy and stability of each clock generator in each of the transmitter and receiver. In other words, errors and low accuracy in the ranging can be caused by a frequency deviation of the oscillator in one of the transmitter and receiver from the oscillator in the other one.
The system described in JP-A-2004-254076 can detect a change in the distance between a transmitter and a receiver, but cannot identify the position of the transmitter. Also, while each of the transmitter and receiver comprises a different clock generator for generating a clock at a predetermined frequency, the accuracy with which a change in distance is identified is affected by the accuracy and stability of each clock generator in each of the transmitter and receiver. In other words, errors in the ranging can be caused by a frequency deviation of the oscillator in one of the transmitter and receiver from the oscillator in the other one.
In the system which utilizes TDOA for positioning described in JP-A-2005-140617, the positioning accuracy is affected by the accuracy with which the time difference of arrival is measured. Generally, an accurate time difference measurement requires a high-speed oscillator and a high-speed counter, causing an increase in power consumption and circuit scale. Also, the time difference measuring accuracy depends on the accuracy and stability of the frequency generated by the oscillator. In other words, a frequency deviation of the oscillator can cause errors in the time difference measurement. However, an accurate and stable oscillator is expensive, resulting in an increased cost of a device which employs such an oscillator.
On the other hand, the positioning/ranging system is required to reduce power consumption, size and cost of component devices. Therefore, the use of a high-speed, accurate, and stable oscillator and a high-speed counter is not preferable for purposes of measuring a time difference with a high accuracy.